Means for focusing a varying magnetic field



y 1956 w. D. RYCKMAN, JR 2,747,140

MEANS FOR FOCUSING A VARYING MAGNETIC FIELD Filed April 22, 1953 Inventor: William D. 'Ryckman, Jrr, y 6! 4? M His Attorne United States Patent 2,747,140 MEANS FOR FOCUSING A VARYING MAGNETIC FIELD William D. Ryckman, Jr., Buechel, Ky., assignor to General Electric Company, a corporation of New York Application April 22, 1953, Serial No. 350,489 2 Claims. (Cl. 315-344) My invention relates to means for focusing a changing magnetic field.

In the utilization of a change in a magnetic field such as produced by a change in current through a solenoid winding the device responsive to the flux change may not be as effective as desired since only a small proportion of the flux or a small number of the total lines of flux are directed to the responsive device. Thus, for example, where a magnetron discharge device is positioned within a solenoid having a diameter which is relatively large compared to the magnetron space charge chamber device, only a very small percentage of the solenoid flux passes through the space charge chamber. Since such spacing is often required, however, due to the presence of the-magnetron envelope and the need for insulating the magnetron from the winding, it is often desirable that the magnetic flux be concentrated so that a larger proportion of it is directed to the space charge chamber in a manner which will increase the magnetronisensitiyity to; sudden solenoid current changes. p

It is therefore a primary object of my invention to provide means for effectively focusing a changing magnetic field. I t i a I It is a further object of my invention-to provide means for rendering a magnetron more sensitive to changes in an applied magnetic field.

Briefly, in accordance with my invention, a changing magnetic field directed through a given cross-sectional area is effectively focused to increase the flux density during field changes within a central portion of the given area by positioning across the magnetic flux path a nonmagnetic conductor having an outline corresponding to a washer or annulus with a relatively small radial segment cut away. Such a conductor may take either the form of a wire loop conforming to that outline or a solid washer with a radial portion removed. When the fiux changes, a current is induced in the conductor to produce an opposing flux change in the area within the conductor outline and an aiding flux change through the annular opening. Since the magnetic field lines are focused or concentrated only during rapid flux changes, the conductive member has no effect on the steady state flux, thus rendering it uniquely useful in cooperation with a direct current magnetron or other responsive device designed to provide a signal or other indication corresponding to rapid flux changes.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

Fig. 1 is a semi-schematic representation of magnetron apparatus incorporating a magnetic focusing device in accordance with my invention;

Fig. 2 is a top view of the apparatus of Fig. 1; and

Fig. 3 is a modification of the magnetic flux focusing device employed in the apparatus of Figs. 1 and 2.

Referring now to Fig. 1, apparatus is shown therein which is designed to provide a signal responsive to sudden increases in current in a high voltage direct current system. A helical coil or solenoid 1 is provided with terminals 2 and 3 adapted to be connected in circuit with the direct current system so that upon flow of current through the solenoid an axially directed magnetic field is established within the coil. To provide a signal responsive to the flux changes within the solenoid caused by sudden changes in the solenoid current a magnetron diode 4 is positioned in the solenoid magnetic field. The magnetron 4 is suitably of a type having a cylindrical thermionic cathode sleeve 5 containing a heater 6 with a thin-walled cylindrical anode 7 surrounding the cathode. These electrodes are spaced with respect to each other by top and bottom mica spacers 8, and are supported by electrode leads which are insulatingly sealed through the base 10 of a tubular glass envelope 9. The magnetron device is conventionally exhausted and sealed. The anode 7 need not be divided into segments although it is preferably split along one side so that it will not act as a short circuited turn which would oppose changes in flux through the space charge chamber defined between the cathode and the anode. The magnetron is positioned within the solenoid, preferably-having itscathode coaxial therewith, and insulated therefrom by a pair of non-magnetic support rods llattachedto insulating annular washers 12 fitted over the tube envelope 9 The operation of such direct current magnetrons is well-known in the art. A suitable source of voltage such as a battery 13 is connected to apply a positive potential to the anode with respect to the cathode, and a heater current source 14 is supplied to the heater to cause the cathode to emit electrons. The electrons emitted from the cathode tend totravel radially to the anode,- but, due to the influence of the axial magnetic field, the electron paths are curved in aplane normal to the magnetron axis. At a critical 'or cutoif magnetic field value the electron orbits are murved sufficiently so that the electrons miss the anode completely and return to the cathode. Accordingly, when the magnetic field reaches this cutoff value the anode current abruptly decreases to produce a signal by changing the voltage drop across a load impedance 15. This impedance may suitably comprise a transformer having its primary winding connected in the magnetron anode circuit for inducing the signal in the transformer secondary circuit. This signal, produced when the anode cur rent either increases or decreases due to the magnetic field passing through the magnetron cutoff value, may be employed in an indicator or control circuit as desired. The application suggested here for such a direct current magnetron as a detector of direct current faults or other current changes exemplifies a type of apparatus in which my invention is profitably employed.

Since the insulation requirements for the solenoid 1 may be very high or for other reasons the cross section area of the magnetron anode 7 is relatively small compared to that of the solenoid, it may be seen that the magnetron sensitivity is adversely affected since only a small proportion of the magnetic field produced can react upon electrons in the magnetron chamber. in accordance with my invention therefore, a conductive washer 16 made of copper or other non-magnetic material is positioned around the magnetron envelope 9 in the vicininty of the space charge chamber and is suitably supported by the magnetron support rods ll. The inner opening in the washer or annulus is large enough Patented May 22, 1956 to admit the tube envelope and the outer diameter is as large as practical without interfering with the solenoid 1 in order that a substantial'proportion of the solenoid cross sectional area is enclosed between the inner and outer diameters of the member 16. As may be seen in Fig. 2, the washer is cut or has a radial portion removed so that it does not constitute a closed single turn linking the flux through the solenoid. The gap 17 thus defined is preferably rather small so that the space charge chamber is substantially encircled by the Washer.

In operation, when a sudden change in fiux occurs due to sudden change in the solenoid current, eddy currents are induced in the washer having the net effect of a current traveling in a closed loop around the outline of the washer. Thus, starting at one edge along the gap 17, such a current flows around the inner periphcry, out along the other edge bordering the gap, around the outer periphery, and back to the starting point. The effect of the induced current is to produce a flux opposing the flux change of the solenoid in the part of the washer closed by its outline whereas a flux aiding the solenoid flux is produced within the inner conductor defined by the annular opening within the washer. The net effect is to concentrate or focus the flux lines in the washer opening, thus causing a higher flux density along the axis of the solenoid and within the space charge chamber during rapid changes of the solenoid flux. .This etfect is exactly the one desired for many applications, the sudden change in the solenoid flux producing an instantaneous response in the magnetron output circuit, which is especially desirable in applications such as directcurrent bus fault detectors. The focusing member 16 has little or no efiect on a steadystate flux or upon a slowly varying one, thus permitting sensitivity to transient currents to be adjusted without affecting the response to steady currents.

Fig. 3 represents a modification of the focusing de vice 16 which operates on the same principles. Here a member 18 is formed from a single closed loop of wire suitably made of copper or other non-magnetic conductor shaped-to conform to the Outline of the split washer 16 of Fig. 2. That is, the closed conducting path defines an annulus having a small radial gap 19. The operation of the device 18 is the same as that of the device 16, previously described, and the mode of construction of the focusing device is usually governed by convenience of manufacture.

While I have shown and described various specific embodiments of my invention it will of course be understood by those skilled in the art that various modifications may be made without departing from the principles of the invention. I, therefore, contemplate by the appended claims to cover any such modifications that fall Within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a direct current magnetron discharge device having a space charge chamber, magnetic means external to said discharge device for providing a magnetic field through said device, and means for focusing the magnetic field flux therein when the field is changing comprising a non-magnetic planar conductor having the outline of an annulus with a small radial sector in the plane of said conductor removed, said conductor surrounding the space charge chamber and positioned within said magnetic means in a plane transverse to the lines of flux.

2. Apparatus for measuring rapid changes in a direct current comprising a solenoid coil connected in circuit with a direct current source, a direct current magnetron discharge device having a space charge chamber comprising a cathode surrounded by an anode positioned Within said solenoid, means for applying a positive potential to said anode with respect to the cathode to provide a direct current flow therethrough, and means for focusing the magnetic field flux produced by said coil when the current in said source changes comprising a nonmagnetic conductor having an outline corresponding to that of an annulus with a small sector removed, said member being positioned within said coil and surrounding said discharge device in an axially transverse plane.

References Cited "in the file of this patent UNITED STATES PATENTS 2,097,- 91 8 Fritz Nov. 2, 1937 2,103,362 Hanseil Dec. 28, 1937 2,151,766 Hollmann Mar. 28, 1939 2,213,543 Brett Sept. 3, 1940 

